Phosphors For Energy Saving and Conversion Technology Vijay B Pawade Download PDF
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Phosphors for Energy Saving
and Conversion Technology
Phosphors for Energy Saving
and Conversion Technology
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efforts have been made to publish reliable data and information, but the author and publisher cannot
assume responsibility for the validity of all materials or the consequences of their use. The authors
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Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and
are used only for identification and explanation without intent to infringe.
Names: Pawade, Vijay B., author. | Dhoble, Sanjay J., 1967- author.
Title: Phosphors for energy saving and conversion technology / Vijay B.
Pawade and Sanjay J. Dhoble.
Description: First edition. | Boca Raton, FL : CRC Press/Taylor & Francis
Group, 2018. | “A CRC title, part of the Taylor & Francis imprint, a
member of the Taylor & Francis Group, the academic division of T&F Informa
plc.” | Includes bibliographical references and index.
Identifiers: LCCN 2018014016| ISBN 9781138598171 (hardback : acid-free paper)
| ISBN 9780429486524 (ebook)
Subjects: LCSH: Electroluminescent devices--Materials. | Solar
cells--Materials. | Phosphors.
Classification: LCC TK7871.68 .P39 2018 | DDC 621.32--dc23
LC record available at https://lccn.loc.gov/2018014016
Preface.......................................................................................................................xi
About the Authors............................................................................................... xiii
v
vi Contents
2.6.8 Ho3+...........................................................................................64
2.6.9 Tm3+...........................................................................................65
2.7 Energy Transfer in Rare Earth Ions................................................... 66
2.7.1 Theory of Energy Transfer.................................................... 67
2.7.2 Excitation by Energy Transfer............................................... 70
2.8 Photoluminescence.............................................................................. 71
2.8.1 Rare Earth–Activated Phosphor........................................... 71
2.9 Classification of Materials................................................................... 72
2.9.1 Metals....................................................................................... 72
2.9.2 Ceramics................................................................................... 73
2.9.3 Polymers................................................................................... 73
2.9.4 Composites............................................................................... 74
2.9.5 Twenty-First-Century Materials........................................... 75
2.9.5.1 Smart Materials....................................................... 75
2.9.5.2 Nanomaterials......................................................... 76
2.10 Applications of Phosphors..................................................................77
2.10.1 Fluorescent Lamps..................................................................77
2.10.2 Light-Emitting Diodes (LEDs).............................................. 78
2.10.3 Flat Panel Displays (FPDs)..................................................... 79
2.10.4 Cathode Ray Tubes (CRTs).....................................................80
2.10.5 PV Technology........................................................................ 81
2.10.6 Upconversion Phosphor......................................................... 81
2.10.7 Downconversion Phosphor................................................... 82
2.11 Types of Host Material........................................................................83
2.11.1 Tungstates................................................................................83
2.11.2 Vanadates.................................................................................84
2.11.3 Oxides....................................................................................... 85
2.11.4 Perovskites............................................................................... 85
2.11.5 Aluminates.............................................................................. 86
2.11.6 Borates...................................................................................... 87
2.11.7 Silicates..................................................................................... 88
2.11.8 Aluminosilicates..................................................................... 89
2.11.9 Fluorides................................................................................... 91
2.11.10 Nitrides..................................................................................... 91
2.12 Other Materials for LED Applications.............................................. 92
2.12.1 Organic LEDs.......................................................................... 92
2.12.2 Polymer LEDs.......................................................................... 93
2.12.3 Quantum Dots for LEDs........................................................ 93
References........................................................................................................ 94
Over the past few years, many universities and colleges have introduced
undergraduate and postgraduate courses on green and renewable energy
sources and the need for sustainable development considering the adverse
effect of globalization on human health and the environment. Therefore,
to sustain the environment for the betterment of mankind, it is essential to
adopt the concept of “Go green.” Currently, this area is gaining more impor-
tance and attracting students and researchers due to its necessity in the
21st century. This book provides basic knowledge about the advantages of
phosphors in energy saving and conversion devices and the scope of envi-
ronmentally friendly technology to save energy and the environment. This
book is very useful in finding alternative renewable and sustainable energy
sources to replace the existing fossil fuel–based polluting technology used
in energy generation. The use of clean and environmentally friendly tech-
nology really helps to reduce the level of harmful pollutants and green-
house gases in the environment and to save our lovely planet. This textbook
is divided into four parts: the first introduces the fundamentals of atoms
and semiconductors, the second deals with an overview of phosphors, the
third introduces the scope of phosphors in the development of light-emit-
ting diodes and photovoltaic technology, and the last gives a brief account
of the role of energy-efficient technology in sustainable development. Based
on these four different parts, the book is further divided into seven chap-
ters. All the chapters were written in a sequential manner and linked to
each other for better understanding. The student is encouraged to expand
on the topics discussed in the book by reading the references provided at
the end of each chapter. Each chapter is written in such a way as to fit the
background of renewable and sustainable energy sources and clean tech-
nology. The content given in this book can be covered in the undergraduate
and postgraduate syllabi of different courses.
Finally, we are grateful to the researchers and publishers who have permit-
ted and provided data wherever necessary to enhance the depth of the book.
xi
About the Authors
xiii
Part I
Fundamentals of Atoms
and Semiconductors
1
Short Review of Atomic and
Semiconductor Theory
1.1 Atomic Theory
In the sciences, especially chemistry, physics, materials science, and elec-
tronics, atomic theory is a scientific theory that provides basic knowledge
about matter, which is composed of tiny units called atoms.
This section gives a short description of the atom. We know that the atom
may be considered as the smallest unit of matter that can have different
chemical and physical properties [1]. Matter is composed of atoms and mol-
ecules. It has four different states: solid, liquid, gas, and plasma, which is
the fourth state of matter and consists of neutral and ionized atoms. The
typical size of an atom is very small, approximately 100 pm [2, 3]. The atom
has no definite size or boundaries, but there are different ways to define the
size of an atom. The behavior of a small atom is well understood using the
concepts of classical physics. Quantum theory has played an important role
in the development of atomic theory and models, and it is also applicable to
exploring and predicting the nature of an atom. In basic science, the word
atom is a philosophical concept from ancient Greece, and it was discovered
in the nineteenth century in the field of modern chemistry that matter is
indeed composed of atoms. Chemists used the word atom in connection with
the study of new chemical elements. Later, in the twentieth century, various
experiments were carried out based on electromagnetism and radioactiv-
ity, for example. Physicists explained that an atom is nothing but a combi-
nation of various subatomic particles, which can exist separately from each
other. The atomic structure consists of a nucleus at the center, formed by the
combination of protons and neutrons, also called nucleons, bonded with one
or more electrons around the nucleus of the atom. According to the litera-
ture, the nucleus makes up 99.94% of the mass of an atom. Protons, which
lie inside the nucleus, carry a positive charge, and electrons have a negative
charge, such that the atom is electrically neutral. Atoms can be attached to
each other by chemical bonds to form a compound. The ability for an atom
to associate or dissociate is responsible for the physical changes observed in
nature, so it is directly linked with the branch of chemistry.
3
4 Phosphors for Energy Saving and Conversion Technology
1.1.1.2 Rutherford Model
This model was discovered by Ernest Rutherford in 1909. He concluded that
the plum pudding model of the atom discovered by J. J. Thomson was not
correct. Thus, in 1911, Rutherford worked on the Thomson model [8], and
with the help of a gold foil experiment, stated that the atom has a small and
heavy nucleus. Later, he designed an experiment in which he used α-particles
emitted from a radioactive substance as a probe to investigate the unseen
atomic structure. According to this experiment, he predicted that if the beam
of emitted α-particles was bombarded and passed in a straight line through
the gold foil, then the Thomson model would be correct. But it was found
that although most of the radioactive α-rays passed through the gold foil,
Short Review of Atomic and Semiconductor Theory 5
• An electron moving around the nucleus does not influence the scat-
tering of α particles.
• In many atoms, the positive charge is situated at the center of the
atom in a relatively small volume called a nucleus. The magnitude of
charge and mass are proportional to each other. Therefore, the con-
centrated central mass and charge of an atom cause the deflection of
α and β particles.
• Elements of high atomic mass do not deflect high-speed α particles,
which carry high momentum compared with electrons.
• The nucleus is about 105 times smaller than the diameter of the atom.
This is like putting a grain of sand in the center of a football [10, 11].
1.1.1.3 Bohr Model
In atomic theory, the Rutherford–Bohr model, also called the Bohr atomic model,
was discovered in 1913. Bohr predicted that the atom has a small, positively
charged nucleus and the electrons travel in a circular orbit around it, similarly
to a solar system. This is due to the presence of electrostatic and gravitational
forces of attraction. The Rutherford model was further improved using the
quantum physical interpretation of the atom. The modern mechanical model
of the atom has been developed following the Bohr atomic model. According to
the laws of mechanics, an electron revolving around the nucleus should release
electromagnetic radiation, and due to loss of energy from the electron, finally, it
will spiral inward toward the nucleus. Hence, as the orbit becomes smaller and
faster, the frequency of radiation should increase with the emission of radiation.
Therefore, it would release continuous electromagnetic radiation. However, in
the nineteenth century, an experiment was performed with electric discharge
showing that atoms emit electromagnetic radiation at certain frequencies.
Therefore in 1913, Bohr proposed a different model, known as Bohr’s model,
to overcome the drawbacks of atomic theory.
Bohr proposed that electrons have classical motion:
Each orbit has fixed energy, called the energy level of an atom. In
this case, the acceleration of the electron does not emit radiation, but
the energy loss of an electron results from classical electromagnetic
radiation.
3. Therefore, an electron in an atom can lose or gain energy when it
jumps from a lower to a higher orbit by absorbing or emitting radia-
tion of certain frequencies.
nλ = 2πr
from those proposed by Bohr, considering that there may be more than
one value of k. Thus, Sommerfeld gave a reason for the fine spectral lines
of the hydrogen-like atom, and it was shown that the frequencies of some
fine spectral lines were in good agreement with the frequencies reported
by Sommerfeld. To explain the fine structure of spectral lines, Sommerfeld
made two corrections to Bohr’s theory:
1. It cannot account for the exact number of lines observed in the fine
spectra of hydrogen.
2. It is unable to define the distribution and arrangement of electrons
in atoms.
3. It fails to discuss the fine spectra of alkali metals (sodium, potas-
sium, etc.).
4. It cannot explain the Zeeman and Stark effect of an atom.
5. It is unable to provide any explanation for the intensities of hydro-
gen’s spectral lines.
Electron
r
N f
FIGURE 1.1
Sommerfeld atomic model.
Short Review of Atomic and Semiconductor Theory 9
1.2 Basics of Semiconductors
Materials that have conductivity lying between conductors and insulators
are called semiconductors. They are always in the form of crystalline or amor-
phous solids having different electrical characteristics [13]. They have high
resistance, which is higher than that of other resistive materials but much
lower than that of insulators. The resistivity of semiconductors decreases
with increasing temperature, meaning that they have opposite behavior
to that of a metal. The conducting properties of a semiconductor can be
10 Phosphors for Energy Saving and Conversion Technology
altered by doping an impurity atom into its crystal structure, due to which
the resistance of the semiconductor is reduced, and hence, it forms a junc-
tion between two different doped regions. At the junction, a current flows
due to charge carriers such as electrons, holes, and ions. Semiconductor
devices have special features, such as passing current more easily uni-
directionally than other devices; also, they have variable resistance and are
sensitive to light or heat energy. As discussed earlier, the electrical charac-
teristics of a semiconductor can be improved by doping a suitable impurity
ion, and components fabricated from such materials have many advantages
in amplification, switching, and energy conversion, for example [14]. The
doping impurity in a semiconductor mostly enhances the charge carriers
present inside the crystal. If the semiconductor is filled with mostly free
holes or positive charge carriers, it is called p-type, and if it is filled with
mostly free electrons or negative charge carriers, it is known as an n-type
semiconductor. Thus, the semiconductor device is fabricated by the combi-
nation of p- and n-type materials, and the formation of a depletion region
between the p- and n-region is responsible for the conduction of electrons
and holes from one region to another. Today, most semiconductor materials
are widely used in electronic devices and electrical components. Also, some
pure elements and compounds have shown excellent semiconductor prop-
erties; these include silicon, germanium, gallium, and so on. However, the
electrical properties of these semiconductor materials, such as Si, Ge, and
GaAs, lie in between those of “conductors” and “insulators.” They are nei-
ther good conductors nor good insulators; hence, they are called semicon-
ductors. Semiconductor materials contain very few valence electrons, and
the atoms are closely packed to form a desired crystalline pattern called a
crystal lattice, in which the valence electrons are able to move only under
certain conditions. Therefore Si and Ge are considered pure or intrinsic
semiconductors, because they are chemically pure, and their conductivity
is controlled by adding an impurity. The impurity is of two types, donor
and acceptor, and by adding it, a number of free electrons or holes can be
generated. The properties of semiconductor materials were first studied in
the middle of the nineteenth century and also in the first decades of the
twentieth century. In 1904, semiconductors were first practically used in
electronic devices and integrated circuits (1958) and widely used in early
radio receivers [15].
1.2.1 Properties of Semiconductors
Among the different conducting materials, semiconductor materials show
excellent electrical and optical properties, which are the unique features of
semiconductor devices and circuits. So, these properties can be very well
understood by knowing their electrical conductivity, resistivity, thermal
conductivity, absorption and emission bands, interband and direct transi-
tions, and so on.
Short Review of Atomic and Semiconductor Theory 11
1.2.1.1 Electrical Properties
The electrical properties of semiconductors can be studied on the basis of
the resistivity of the materials. For example, metals, such as gold, silver, and
copper, have low resistance and are good conductors of electricity, whereas,
materials such as rubber, glass, and ceramics carry a high resistance and
are unable to conduct electricity. If a semiconductor does not contain any
impurities, it cannot conduct an electric current; only by the addition of a
suitable impurity can it work as a conductor. Semiconductors made up of
only a single compound or element are called elemental semiconductors (e.g.,
silicon). On the other hand, if they are made by the combination of two or
more compounds, they are called compound semiconductors (such as GaAs,
AlGaAs, etc.). Hence, semiconductors constitute a large class of materials
having resistivities lying between conductors and insulators. Their resistiv-
ity varies over a wide range, and it is reduced by increasing the temperature,
as shown in Figure 1.2. Among the different semiconductor elements, silicon
(atomic number 14) and germanium (atomic number 32) have the most prac-
tical importance, because they have four valence electrons in their outermost
shell. Both have a tetrahedral structure, and each atom shares one valence
electron with the neighboring atom, forming a covalent bond between them.
Therefore, the electrical properties of semiconductor materials reveal that
Temperature (T)
FIGURE 1.2
Temperature dependence of resistance in semiconductors.
12 Phosphors for Energy Saving and Conversion Technology
The following list briefly describes some important terms related to the
electrical properties of materials.
1.2.1.1.1 Thermal Conductivity
This is the property of a material to transfer a flow of heat from one end to
another. It is expressed in the form of the law of heat conduction, also called
Fourier’s law of heat conduction. The rate of heat transfer is lower across a mate-
rial having low thermal conductivity than one with high thermal conductiv-
ity. Materials with high thermal conductivity can be applied as heat sinks,
and those with low thermal conductivity are used as thermal insulators in
most electrical equipment. Therefore, this property of the materials depends
on temperature. Thermal resistivity is defined as the reciprocal of thermal
conductivity. In general, metals, such as copper, aluminum, gold, and silver,
are good conductors of heat; however, materials such as wood, plastic, rub-
ber, and so on are poor heat conductors, so-called insulators.
1.2.1.1.2 Electrical Conductivity
This is the measure of the capacity of electrical current flow through the
material. It is also referred to as specific conductance. Put another way, electri-
cal conductivity is defined as the reciprocal of electrical resistivity. The elec-
trical conductivity of the conductor increases as the temperature decreases.
In many semiconducting materials, the conduction occurs via charge carriers
such as electrons or holes. Therefore, metals (e.g., silver) are materials with
good electrical conductivity, whereas insulators, such as glass, pure water,
and so on, have poor electrical conductivity. Another example is diamond,
which is an electrical insulator due to its regular and periodic arrangement
of atoms but is conductive of heat via phonons.
TABLE 1.1
Bandgap and Conductivity of Some Semiconductor Materials
Semiconductor Band Conductivity
Sr. No Material Gap (eV) (Ω−1-m−1)
1 Si 1.11 4 × 10−4
2 Ge 0.67 2.2
3 GaP 2.25 2.2
4 GaAs 1.42 1 × 10−6
5 InSb 0.17 2 × 104
6 CdS 2.40 2 × 104
7 ZnTe 2.26 2 × 104
Short Review of Atomic and Semiconductor Theory 13
1.2.1.1.3 Variable Conductivity
In their normal state, semiconductors act as poor conductors. To start the flow
of current in a semiconductor, therefore, requires an electrical or heat supply
to its junction, and we know that valence bands (VBs) in semiconductors are
filled with free electrons. There are different ways to make the semiconduc-
tor a conducting material by doping or gating. The n-type or p-type junc-
tion of the semiconductor refers to the shortage of electrons. The current flow
through the conductor is due to the unbalanced number of electrons [16].
1.2.1.1.4 Heterojunctions
These types of junction are formed by a combination of two different types of
doped semiconducting materials. Generally, they consist of p- and n-doped
germanium. The junction shows the movement of charge carriers, such as
electrons and holes, between two doped semiconducting materials. The
n-doped region of germanium is filled with negatively charged electrons,
while its p-doped region would have an excess of positively charged holes.
The motion of electrons and holes proceeds by a recombination process,
which allows a migration of negatively charged carriers (electrons) from
the n-type to combine with the positively charged carriers (holes) from the
p-type. Finally, this motion of charge carriers from one region to another
results in an electric field [16, 17].
1.2.1.1.5 Excited Electrons
Usually, a greater number of electrons are present in the material when it is
in the state of thermal equilibrium. A difference in electric potential would
cause a semiconducting material to leave the state of thermal equilibrium
and develop a non-equilibrium state. Due to this, electrons and holes inter-
act via ambipolar diffusion. When the thermal equilibrium is disturbed in
a semiconducting material, the density of electrons and holes changes. This
results in a temperature difference or photons, which enter the system and
form electrons and holes. Therefore, this process, which forms and anni-
hilates the present electrons and holes, is referred to as the generation and
recombination process [16].
1.2.1.1.6 Light Emission
Light emission from semiconductor materials occurs due to the movement of
an electron from the lower to the most excited states. Then, it emits light by
acquiring a lower energy state without producing a nonradiative transition
[18]. Such light-absorbing or -emitting semiconductor materials are used for
the fabrication of light-emitting diodes (LEDs) and display devices.
1.2.1.2 Optical Properties
The optical properties of semiconductors involve the interaction of elec-
tromagnetic radiation with atoms, which constitutes different processes,
such as absorption, emission, reflection, refraction, diffraction, and so on.
Electromagnetic radiation is important for the different quantum processes,
which shows interesting optical properties of semiconductors.
Optical spectroscopy is an important field in the area of science and tech-
nology, which gives us knowledge about the structure and properties of
matter (atoms or molecules) based on their spectroscopic characterization
[20–25]. Line spectra of atoms were studied in the eighteenth and nineteenth
centuries; they provide a great deal of information about the structure and
electronic energy levels of the atom [26–28]. Similarly, optical spectroscopy
of semiconductors is important in studying the behavior of semiconductor
materials after exposure to light radiation. Figure 1.3 shows the band gaps
and emission wavelengths of some semiconductor materials. In the early
1950s, detailed knowledge was obtained from semiconductors on the vari-
ous eigenstates, impurity and defect levels, energy bands, electron excitonic
levels, density of states, width of the energy level, symmetries, and so on.
The optical properties of semiconductors basically depend on the nature of
the electronic band structures and are also related to the lattice structure
and bonding between the atoms. Therefore, the lattice symmetry and space
groups of an atom are also important to define the structure of energy bands.
The optical properties of semiconductors are further classified into electronic
and lattice properties, in which electronic properties correspond to the elec-
tronic states of the semiconductor, while the lattice properties involve the
absorption and creation of phonons during vibrations of the lattice. Thus, the
Yellow
Green
Violet
Blue
Red
Eg (eV )
0 1 2 3 4
l (mm)
75 3 2 1 0.5 0.35
FIGURE 1.3
Band gaps and emission wavelengths of some semiconductor materials. (From http://nptel.
ac.in/courses/117102061/LMB2A/3a.htm)
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« Le sergent de l’intendance me dit, du haut de son éléphant :
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« Il avait abaissé sa grosse tête entre ses grosses pattes de
devant, qui étaient croisées comme celles d’un petit chat. Il avait l’air
de l’innocence et de la désolation personnifiées, et par parenthèse
sa grosse lèvre inférieure poilue tremblotait et il clignait des yeux
pour se retenir de pleurer.
« — Pour l’amour de Dieu, que je dis, oubliant tout à fait que ce
n’était qu’une bête brute, ne le prends pas ainsi à cœur ! Du calme,
calme-toi, que je dis. (Et tout en parlant je lui caressai la joue et
l’entre-deux des yeux et le bout de la trompe.) Maintenant, que je
dis, je vais bien t’arranger pour la nuit. Envoyez-moi ici un ou deux
enfants, que je dis au sergent qui s’attendait à me voir trucider. Il
s’insurgerait à la vue d’un homme.
— Tu étais devenu sacrément malin tout d’un coup, dit Ortheris.
Comment as-tu fait pour connaître si vite ses petites manies ?
— Parce que, reprit Térence avec importance, parce que j’avais
dompté le copain, mon fils.
— Ho ! fit Ortheris, partagé entre le doute et l’ironie. Continue.
— L’enfant de son mahout et deux ou trois autres gosses des
lignes accoururent, pas effrayés pour un sou : l’un d’eux m’apporta
de l’eau, avec laquelle je lavai le dessus de son pauvre crâne
meurtri (pardieu ! je lui en avais fait voir) tandis qu’un autre extrayait
de son cuir les fragments des carrioles, et nous le raclâmes et le
manipulâmes tout entier et nous lui mîmes sur la tête un
gigantesque cataplasme de feuilles de nîm [8] (les mêmes qu’on
applique sur les écorchures des chevaux) et il avait l’air d’un bonnet
de nuit, et nous entassâmes devant lui un tas de jeunes cannes à
sucre et il se mit à piquer dedans.
[8] Nom d’une plante de l’Inde.
Mulvaney se tut.
— Et après ? demandai-je.
— Vous le devinez, reprit Mulvaney. Il y eut confusion, et le
colonel me donna dix roupies, et le commandant m’en donna cinq, et
le capitaine de la compagnie m’en donna cinq, et les hommes me
portèrent en triomphe autour de la caserne.
— Tu es allé à la boîte ? demanda Ortheris.
— Je n’ai plus jamais entendu parler de mon malentendu avec le
pif de Kearney, si c’est cela que tu veux dire ; mais cette nuit-là
plusieurs des gars furent emmenés d’urgence à l’ousteau des Bons
Chrétiens. On ne peut guère leur en faire un reproche : ils avaient eu
pour vingt roupies de consommations. J’allai me coucher et cuvai les
miennes, car j’étais vanné à fond comme le collègue qui reposait à
cette heure dans les lignes. Ce n’est pas rien que d’aller à cheval sur
des éléphants.
« Par la suite je devins très copain avec le vénérable Père du
Péché. J’allais souvent à ses lignes quand j’étais consigné et
passais l’après-midi à causer avec lui : nous mâchions chacun notre
bout de canne à sucre, amis comme cochons. Il me sortait tout ce
que j’avais dans mes poches et l’y remettait ensuite, et de temps à
autre je lui portais de la bière pour sa digestion, et je lui faisais des
recommandations de bonne conduite, et de ne pas se faire porter
sur le registre des punitions. Après cela il suivit l’armée, et c’est ainsi
que ça se passe dès qu’on a trouvé un bon copain.
— Alors vous ne l’avez jamais revu ? demandai-je.
— Croyez-vous la première moitié de l’histoire ? fit Térence.
— J’attendrai que Learoyd soit de retour, répondis-je
évasivement.
Excepté quand il est soigneusement endoctriné par les deux
autres et que l’intérêt financier immédiat l’y pousse, l’homme du
Yorkshire [10] ne raconte pas de mensonges ; mais je savais Térence
pourvu d’une imagination dévergondée.
[10] Learoyd.
Clark Russell.